Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface; and,

FIG. 2 is a front view of a second embodiment thereof.

The broken line showing of text and graphic elements within the graphical user interface form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 